Friday, April 18, 2008

Boeing's Composite Problem

Structural weakness forces Boeing to retrofit the 787 Dreamliner.

By David Talbot

Boeing says that its 787 Dreamliner--a midsize, fuel-efficient passenger jet that is currently in development--will be the first commercial aircraft in which major structural elements are made of composite materials rather than aluminum alloys. The changes are expected to slash component weights 20 percent, significantly boosting fuel efficiency.

Such composite materials--layers of superstrong carbon fibers and epoxy--have long been used in military jets, where money is rarely an object, and in commercial jets for parts like luggage-rack frames. But Boeing is learning how hard composites can be to analyze effectively and build economically for commercial jet structures. The company has had to delay the 787's introduction because elements of the composite-made wing box--the major structure inside each wing--buckled in stress tests.

The wing box begins in roughly the middle of the plane and extends about two-thirds of the wingspan. This key component--more than 15 meters long and 5 meters wide--was designed and built by Boeing together with Mitsubishi Heavy Industries and Fuji Heavy Industries, in Japan. Pat Shanahan, vice president for the 787 program, said in a conference call last week that structural testing had "identified the need to stiffen elements within the center wing box."

The fix requires adding new brackets and other parts to wing boxes already built, as well as modifying the design of boxes not yet built. The retrofits of existing boxes will intrude into wiring pathways, compounding the problems. So Boeing is pushing back the 787's delivery date about six months, from the first quarter to the third quarter of 2009.

The issue with composites isn't that they aren't strong; it's that they are so internally complex. They consist of layers oriented in different directions; those layers, in turn, are made of individual fibers that may vary somewhat in composition. This makes it difficult for engineers to accurately mimic their performance in computer models for premanufacture testing.

"Composite materials are more difficult to analyze than simple homogenous metals," says John Hansman, director of the International Center for Air Transportation, at MIT. "You generally don't model every fiber in the structure, so you come up with models that have simplifications."

Prayer and a wing: A preproduction Boeing 787 was unveiled last year, but parts of its wing box (above)--the major structure of each wing--buckled in stress tests. The structure, made of composite materials, measures more than 15 meters by 5 meters and weighs 55,000 pounds, including testing hardware and instruments. The company is stiffening this version as it tweaks the design of future wing boxes. Credit: Boeing

What's more, he says, composites allow engineers to make custom shapes, but these custom shapes compound the already difficult modeling problem. "You have many more design options, which can be both a strength and a weakness. There are many more things I can do with composite materials--add strength in specific places, take it away--but then you have combinations of both the geometry and the particular layup of the composite materials" that are unique.

Boeing's mechanical stress tests start with representative pieces (known as coupons), then move on to progressively larger parts of the structure, and finally to the full structure. Boeing puts the structural parts into huge hydraulic machines that bend and twist them to mimic stresses that go far beyond worst-expected conditions in real flights. It was during such tests that problems emerged with structural spars in the wing box.

Shanahan said in last week's conference call that Boeing has traced the problem back to an error in earlier modeling analysis, but he did not explain the details. "We discovered it. We'll go back and correct it," he said. Shanahan added that Boeing has not lost faith in its decisions to more widely use composites; 95 percent of thousands of tests have yielded as-good or better-than-expected results. On one such test--of the composite-made fuselage "barrel"--engineers had to stop the test for fear of breaking the test equipment, he bragged.

David Roylance, a composites expert and associate professor in materials engineering at MIT, says that Boeing's experience with the 787 shows that the industry is still on a learning curve in using composites more widely in commercial planes. "There are a whole variety of things with composites that are engineerable, but are different than metals," he says. "So it takes time for people to feel comfortable with it."

FOREVER BRUCE LEE

About Me

Great Minds Have Similar Thoughts

Champions aren't made in gyms, champions are made from something they have deep inside them - a desire, a dream, a vision. They have to have last-minute stamina, they have to be a little faster, they have to have the skill and the will. But the will must be stronger than the skill.-Muhammad AliI'll be more enthusiastic about encouraging thinking outside the box when there's evidence of any thinking going on inside it.- Terry PratchettNot to be absolutely certain is, I think, one of the essential things in rationality.- Bertrand RussellWhat we think, or what we know, or what we believe is, in the end, of little consequence. The only consequence is what we do.Sometimes what's right isn't as important as what's profitable.- Trey Parker and Matt StoneThere are only two kinds of people who are really fascinating: people who know absolutely everything, and people who know absolutely nothing.- Oscar WildeSometimes I lie awake at night, and I ask, "Where have I gone wrong?"/ Then a voice says to me, "This is going to take more than one night."- Charles M. SchulzThere is nothing worse than aggressive stupidity.- Johann Wolfgang von GoetheThe significance of man is that he is insignificant and is aware of it.- Carl BeckerA lie can travel halfway around the world while the truth is putting on its shoes.- Mark Twain"If you know how to spend less than you get, you have the philosopher's stone." So said Benjamin Franklin more than 200 years ago. How much easier it is to be critical than to be correct.- Benjamin DisraeliOf course the game is rigged. Don't let that stop you--if you don't play, you can't win.- Robert HeinleinAbility will never catch up with the demand for it.- Malcolm ForbesNo man remains quite what he was when he recognizes himself.- Thomas MannNo man needs a vacation so much as the man who has just had one.- Elbert HubbardThere is no pleasure in having nothing to do; the fun is in having lots to do and not doing it.- Mary Wilson LittleBooks to the ceiling,/ Books to the sky,/ My pile of books is a mile high./ How I love them! How I need them!/ I'll have a long beard by the time I read them.- Arnold LobelLeif Ostling said in a statement that his comments about Germany had been "interpreted in a way that was not intended."If a man will begin with certainties, he shall end in doubts; but if he will be content to begin with doubts he shall end in certainties.- Sir Francis Bacon"It's not the voting that's democracy, it's the counting."- Tom StoppardElections are won by men and women chiefly because most people vote against somebody rather than for somebody.- Franklin P. AdamsInvention is the mother of necessity.- Thorstein VeblenDon't try to solve serious matters in the middle of the night.- Philip K. Dick